The use of scintillation cameras for medical diagnosis in nuclear medicine is an established field of medical instrumentation technology. A form of scintillation camera employing a detector head utilizing a planar scintillation crystal for detecting the occurrence of radioactive emissions and having an image recordation means for storing information representing the interaction of radioactive emissions relative to the scintillation crystal in a two-dimensional coordinate system encompassing the crystal is described in U.S. Pat. No. 3,057,011. The advent of such scintillation cameras complemented to some extent and displaced to a large extent the use of scintillation scanners. Conventional radioisotope scanning is described at pages 27 and 28 of the article "Radiation and Medicine" published in the January/February, 1965 issue of Atomics, Volume 18, Number 1. In addition to more conventional scanners, scintillation cameras have even been used as scanning devices for particular purposes. A scintillation camera used for tomographic scanning, for example, is described in a University of California Report UCRL-16899, published May 31, 1966. A variation of the scanning scintillation camera is a scanning platform. Such a system is described in an article by Michael B. D. Cooke and Ervin Kaplan, entitled "Whole-Body Imaging and Count Profiling with a Modified Anger Camera", Journal of Nuclear Medicine, Vol. 13, No. 12, December, 1972, pp. 899-907. In the utilization of a scanning platform, a patient is positioned on the platform and the platform is moved laterally beneath a fixed scintillation camera detector head.
The object of the present invention is to alleviate certain problems and difficulties that exist in conventional systems. Using the overhead gantry system of this invention, a great deal less room is required for whole body imaging as compared with a scanning platform arrangement. That is, it is necessary to move only a scintillation detector head approximately 18 inches in diameter over an area having dimensions of approximately 8 feet by 4 feet. This is in contrast to the required motion of a scanning platform which necessitates moving the entire area of a 4 foot by 8 foot platform past a fixed scintillation camera detector head. Also, it is far easier for hospital personnel to attend to a patient situated in a stationary position than on a moving table. The motion of a scanning platform further poses obstacles to the positioning of equipment or instruments proximate to a patient on the platform. Using the present invention, the difficulties cited are obviated while retaining the capability for whole body imaging.
The preferred embodiment of the invention illustrated herein provides several further advantages. A portion of the conventional disc-shaped scintillation crystal is masked so that only a rectangular section of the scintillation crystal contributes recordable information. This ensures a uniformity of contribution to the image produced from all areas of a patient which are scanned by eliminating the integration of detected radioactive events from areas where non-uniform overlapping viewing would otherwise occur using a circular or disc-like scintillation detector. With essentially the same consideration in mind, the emission registration portion of the scintillation detector is passed entirely beyond the boundary limits of the field of interest when the detector head undergoes a change in direction of lateral translational motion during scanning. The peripheral portions of the field of interest are thereby exposed to the emission sensitive portion of the scintillation detector to the same extent as are the interior portions of the field of interest. Thus the imaging information derived from the peripheral portions of the field of interest has the same reliability as information derived from more central regions of the field of interest.
Preferably, the scanning mechanism of this invention is provided with an automatic speed control which is regulated by a feedback signal derived from the position sensors. It is thereby possible to balance the requirement for maximizing the number of detected radioactive events with the desirability of concluding an imaging study as rapidly as possible. Speed is maintained at a uniform pace within the viewing area, but the detector head is cushioned in its movement by controlling the acceleration and deceleration profiles both at the beginning and at the termination of any scanning pass of the detector through the viewing area. In this manner, position information inaccuracy in imaging due to mechanical backlash is minimized.
Another advantageous feature of the peferred embodiment of the invention is the ability to adjust the spacing between scanning passes. This is accomplished by providing an indexing selection by which the emission registration portion of the scintillation crystal views overlapping, contiguous, or separated segments of the field of view when moving in adjacent scanning paths. There is also a scanning pass control associated with the index selection unit. The scanning pass control determined the number of scanning passes which the detector head will traverse. Moreover, in conjunction with the variable dimensions of the field of view from which information may be tabulated, automatic adjustments are made to the displayed image so that an undistorted image of maximum size will be produced for any dimensions of the field of view selected. This is advantageously accomplished by automatically associating the proper degree of minification with the dimensions of the field of view selected.
Certain features of safety are provided as elements of the preferred embodiment of the invention. These include a telescoping detector head suspension unit provided with a balancing arm which automatically adjusts for variations in the weight of the detector head. Such variations typically arise since interchangeable collimators of different weight are selectively attached to the detector head. Also, the translation inducing portion of the scanning mechanism is provided with clutch assemblies so that excessive resistance to movement of the detector head will cause disengagement of the motion inducing mechanism. Furthermore, an electrical sensing device in the system limits maximum motor torque in the translation inducing mechanism.